This invention relates to a cylinder block for an axial piston pump or motor that is subjected to a unique manufacturing process to provide piston bores that do not require liners.
Axial piston pumps and motors are used in various applications in the aerospace industry. These pumps and motors are subjected to severe load requirements and often operate in harsh environmental conditions. Each pump and motor traditionally includes a cylinder block with a plurality of piston bores formed therein that each receive a piston in a reciprocating relationship. The block is coupled for rotation which a shaft and works in conjunction with the reciprocating pistons to operate as a pump or motor.
The loading and environmental conditions in which these pumps and motors operate exert high levels of stress on the cylinder block and pistons. High stress levels can result in increased piston bore wear and can ultimately lead to cracking or fracture within the cylinder block. To increase reliability and reduce maintenance and service operations it is desirable to have a minimal amount of wear within each piston bore.
Traditionally, the piston bores have inserts installed within each bore to provide an improved frictional surface for the piston. One example is shown in U.S. Pat. No. 3,319,575, which utilizes bushings in each bore to reduce wear. It is difficult to manufacture the bushings to the tight tolerances required for the application. It also increases assembly time and cost to perform multiple insert operations on the cylinder block to install the bushings in the bores. Another example is shown in U.S. Pat. No. 6,180,183, which utilizes a process for casting bronze piston liners in each of the bores. This process is time consuming and expensive.
Thus, it is desirable to provide method for manufacturing a cylinder block that eliminates the need for piston bore inserts or liners. It is also desirable for the process to reduce costs and assembly time in addition to overcoming the above referenced deficiencies.
The subject invention provides a cylinder block for an aircraft pump or motor that includes unlined piston bores. The cylinder block is formed from a predetermined material, such as steel, and includes a plurality of piston bores formed within the cylinder block. Each of the bores has a bore engagement surface having a predefined material surface finish. A piston is mounted within each one of the bores. Each piston defines an engagement piston surface that is in direct sliding contact with the engagement bore surface.
The method for making the cylinder block includes the following steps. The cylinder body is turned to a rough shape. Each of the piston bores are machined to an initial rough shape. The cylinder is heat treated to achieve a desired hardness. Then the bore is finish machined to a predetermined surface finish to define a final bore surface for direct engagement with a piston surface.
Additional steps include attaching a valve plate to one end of the cylinder block. Preferably, the valve plate is diffusion bonded to the cylinder block by applying a predetermined pressure force against the valve plate and heating the valve plate and cylinder block to a predetermined temperature to achieve a predefined bond strength.